1. Field of the Invention
The present invention relates generally to laser-induced refractive index gratings, and more particularly pertains to superimposed fast transient and permanent holographic gratings, and techniques for producing such superimposed gratings.
2. Discussion of the Prior Art
Light scattering from laser-induced refractive index gratings in materials is the physical mechanism underlying many techniques for controlling light beams in devices useful in a variety of opto-electric technology applications. Examples include phase conjugators, holographic information storage, beam switching and amplification, and intracavity modulation of lasers. Currently known laser-induced grating devices are one of two types, materials producing permanent holographic gratings, and materials producing fast transient gratings. Permanent holographic gratings have high light scattering efficiencies but slow response times, while fast transient gratings have low scattering efficiencies but fas response times.
Moreover, Four Wave Mixing (FWM) processes are currently of significant interest in science and technology because of their importance in modern optical applications such as phase conjugation, and also because they provide a powerful spectroscopic tool for probing the properties of the interaction of light and matter.
The physical processes underlying the laser-induced gratings that give rise to FWM signals can be classified in two categories according to their decay times after the laser writing beams have been turned off. The first category is that of transient gratings with fast decay times, which includes thermal gratings, population gratings, and nonlinear mixing due to third order susceptibility. The second category is permanent gratings which remain in a relatively permanent state after the laser writing beams have been turned off. The most common cause of this type of grating is the photorefractive effect involving ionization of a defect, charge migration and charge trapping. Permanent gratings are also referred to as holographic gratings because of their potential use in holographic information storage applications.
However, the prior art generally discloses and teaches gratings which are only transient gratings, and gratings which are only permanent holographic gratings, with each having its own attendant advantages and disadvantages, and has not heretofore disclosed a device which simultaneously provides a superposition of both types of gratings. Moreover, the prior art has generally provided permanent holographic gratings in a variety of crystalline host materials such as lithium niobate, but has not heretofore produced permanent holographic gratings in a glass host, except amorphous semiconductor films called chalcogenide glasses.